The three provisional patent applications referred to above were combined into one patent application, Ser. No. 10/152,024, because they all dealt with two-step chemical precipitation and “field separation” technologies to remove fine metal precipitates from water, which was the focus of parent application Ser. No. 10/152,024. This remains an important aspect of the invention of this continuation-in-part application; however, this application is also directed to other aspects of the invention, as will appear more fully below. Thus, in the following disclosure, two-step precipitation processes for removing heavy metals from water are discussed first, with the other aspects of the invention being discussed later.
The removal of heavy metals from water is an important aspect of water treatment. There are many technologies for accomplishing this; however, one of the most cost effective means is chemical precipitation. “Chemical precipitation”, as used herein and generally in the art, refers to reacting dissolved metals with an additive chemical of some sort so that the metals to be removed are rendered insoluble, so that they can then be separated from the water. Raising the pH to a neutral or an alkaline level will precipitate most heavy metals as metal hydroxides. However, hydroxide precipitation is usually not effective enough to meet strict new discharge limits. Metal hydroxides are not insoluble enough to meet these limits and metal ions that are chelated will not precipitate at all. Therefore, more advanced treatments such as reaction with organic or inorganic sulfides must be used. These chemistries will produce metal sulfides that have lower solubility than hydroxides and will break chelate bonds to allow the metals to precipitate.
The Department of Army Engineering and Design Manual No. 1110-1-4012 on page 2-2 (Precipitation/Coagulation/Flocculation), shows the difference between the solubility of metal hydroxides and metal sulfides. Under ideal conditions, the optimum metal hydroxide solubility ranges from 102 to 10−2 mg/L. Under ideal conditions, the optimum metal sulfide solubility ranges from 10−2 to 10−12 mg/L.
If all the metals (chelated and non-chelated) are precipitated with sulfide chemicals in a one-step precipitation, the removal is complete, but the cost of treatment is high, often prohibitively high for waste streams containing high concentrations of heavy metals. If most of the metals are first removed as metal hydroxides in a first-step precipitation, and the remaining metals are polished out in a second-step precipitation, the removal of metals is improved and the cost of treatment is much lower. This patent application shows that it is beneficial to use selected “field separation” methods that have not been used or contemplated before in combination with this two-step precipitation process.
The concept of removing heavy metals using sulfides and ferrous compounds was described by Anderson in U.S. Pat. No. 3,740,331. However, Anderson fails to suggest refinements and additions provided by the present invention that make this basic technique cost-effective in today's processing environment. Specifically, Anderson does not suggest that removing metals can be performed more efficiently if the heavy metals are removed in a two-step precipitation process. The teachings of the Anderson patent are simply that using ferrous compounds with sulfides will result in better metal removal. No suggestion is made to use “field separation” methods effective in removing fine and fragile metal precipitates.
The fundamental disadvantage of doing a sulfide precipitation according to Anderson is that it produces very fine colloidal particles that are hard to remove. The present inventor attempted to remove these particles with a sand filter or with a one micron sized back-washable filter and was unsuccessful.
Fender, in U.S. Pat. No. 4,422,943, describes the use of iron pyrite as a source of sulfide to precipitate heavy metals as metal sulfides. He also describes the benefits of using a two-step precipitation process. In claim 2, Fender describes the step of separating precipitated sulfides by filtration (specifically, sand filtration), but does not contemplate using the “field separation” methods described in this application. However, to accomplish filtration, he uses a polymer to increase the particle size so the sand filter can remove the metal sulfides. It is known in the art that using an organic polymer to increase the size of the metal sulfide precipitates will cause fouling problems with sand filters. The “field separation” methods covered by this patent application are not subject to fouling, as are filters. Furthermore, sand filters have a limitation on the size of particles that can be removed; even a well designed multi-media sand filter can remove particles only down to about 20 microns in size. Metal sulfide precipitation will produce colloidal-sized particles of less than one micron in size, which will pass through a sand filter.
With the exception of microfiltration which can remove sub-micron sized particles, the present inventor has found no filtration equipment capable of consistently and economically removing fine metal sulfide particles. More specifically, the present inventor has experimented with a back washable filter manufactured by Asahi. It had a plastic-mesh filtering element with a one micron opening size. This was significantly smaller than the metal sulfide precipitates, judged to be at least 30 microns because they were visible to the naked eye. However, even low operating pressures (about 10 psi), were sufficient to deform the shape of the metal sulfide precipitates and force these >30 micron sized particles through one micron sized openings.
The only commonality between this patent application and the Fender patent is they both recognize the economic importance of using a two-stage precipitation process, which is known art. In summary, this application deals with other forms of soluble and insoluble sulfide treatment rather than iron pyrite and “field separation” equipment rather than filters, which is an improvement on the Fender patent. Further, the Fender patent only deals with iron pyrite as a source of sulfide to precipitate heavy metals; the present application deals with other sulfides that are known to produce small metal sulfide particles that are difficult to filter.
The art recognizes a difference between filtration equipment and “field separation” equipment, as discussed in the Chemical Engineering document dated February 1997, Volume 104, Issue 2, Page 66. Filtration equipment includes: straining, cake filtration, deep bed filtration, and membrane filtration and always involves a physical barrier that prevents the passage of particles over a specific size. “Field separation” techniques include gravitational settling, centrifugal settling, hydrocyclone separation, dissolved air flotation, expanded plastics flotation, and magnetic separation. The difference is that filtration involves a physical barrier to capture particles while “field separation” involves force fields, provided by inter-molecular, gravitational, centrifugal, and/or magnetic forces to separate particles from water.
U.S. Pat. No. 6,099,738 to Wechsler deals with a method and system for removing solutes from a fluid using magnetically conditioned coagulation. This method includes the steps of magnetically conditioning the fluid by applying a magnetic field to enhance the precipitation of solute particles for coagulation; adding a coagulant to the fluid before, during, and after application of the conditioning magnetic field to coagulate the increased available solute particles to form colloids; and collecting the colloids for removal from the fluid. Wechsler neither contemplates combining magnetic seeding and polymer addition with a two-step metal precipitation process as a means for efficiently removing heavy metals from wastewater, nor combining magnetic separation principles with gravity settling in one treatment vessel, as described herein.
According to the present invention, any magnetic separation method can be used; however, in the preferred embodiment the magnetic separator used to capture the magnetic particles is mounted in the treatment tank rather than as a separate collection device, which is novel. This approach has three advantages: (1) one less piece of equipment is needed, (2) the system can be cleaned without interrupting the water flow, and (3) permanent magnets can be used rather than electromagnets.
Magnetic seeding is used in some embodiments of the present invention to remove precipitated pollutants and other non-magnetic particles from water. Magnetic seeding is known per se for such purposes. Specifically, the Department of Energy published studies (C. Tsouris, et. al., Electrocoagulation for magnetic seeding of colloidal particles, Physiochem Eng. Aspects (accepted paper) December 1999; C. Tsouris, et. al., Flocculation of paramagnetic particles in a magnetic field, Journal of Colloid and Interface Science, 171, 319-330; T-Y Ying et. al., High-gradient magnetically seeded filtration, Chemical Engineering Science 55 (2000) 1101-1113) addressing the effectiveness of magnetic seeding to remove colloidal sized particles. The DOE investigators studied magnetically seeded solid/liquid separation combining magnetic seeding under turbulent-shear flow conditions with high field gradient magnetic filtration. They concluded that magnetic seeding was effective in removing fine particles. They used seed particle concentration, solution pH, and ionic strength parameters that determine the zeta-potential of particles to significantly affect the particle removal performance. They did not use organic polymers to bind the magnetic seed materials to the low-magnetic particles to enhance removal, and did not apply magnetic seeding and filtration principles to the second step of a two-step metal precipitation process using sulfide precipitants.
Nilsson U.S. Pat. No. 3,980,562 shows an apparatus for magnetic separation, including a device for removal of accumulated particles from magnetic disks used to collect the particles. More specifically, Nilsson teaches that suspended particles and high molecular weight substances can be removed from water by adding a ferromagnetic particulate to the water and using a magnetic field to separate the combined particles. Nilsson also suggests addition of chemical “flocking agents”, giving as example lime, alum, iron chloride, polyelectrolytes and water glass. Col. 1 lines 10-28. These materials are properly referred to as coagulants, in that they affect the charge of the particles, as compared to the polymer flocculants used in practice of the present invention, which attach particles with long chain polymers as discussed below. Nilsson shows collecting the magnetized particles on opposed walls of disks enclosing permanent magnets, and then scraping them off onto a conveyor belt for disposal. A sector of the disks may be provided without magnets, to facilitate the scraping. Col. 5, lines 18-20. In the Nilsson design, the sector provided without magnets is located at the top of the collector magnet disks because this is the location of the conveyor belts removing the scraped magnetite. However, in the present invention, the sector that is free of magnets is located at the bottom of the collector magnet disk so that after the magnetite is scraped off the disk, it settles by gravity to the bottom of the settling chamber (74).
In researching the present work, the present inventor found that a strong enough bond between the magnetic seed material and the non-magnetic metal sulfide precipitates to enable reliable separation could not be achieved unless a flocculating polymer was also used. The polymer binds the magnetic seed material together with the fine metal sulfide particles so they can be removed by a low field strength magnetic separator or by gravity settling.
Another novel approach of this present patent is the removal of fine precipitates formed, for example, in the second step of this two-step precipitation process, by the use of expanded plastics to enhance flotation. The present inventor successfully attached fine metal precipitates to expanded polystyrene (EPS) with a flocculating polymer. The EPS, having the precipitates attached thereto by the flocculating polymer, floats, allowing the metal precipitates to be removed from the waste stream. The same “EPS flotation” technique of the invention can be used to remove particulates from other water streams, e.g., remove dirt from stormwater runoff.
The concept of enhanced flotation using highly buoyant EPS is similar to the principle used in dissolved air flow (DAF) equipment. DAF uses micro-bubbles to float fine particles out of water, while (in at least one embodiment) the present invention uses an expanded plastic like EPS; this eliminates the energy cost involved with compression of air to form the micro-bubbles.
To date, two-step precipitations have been rarely used because they require additional equipment and space. This level of treatment was not necessary because existing regulatory limits could be achieved with a one-step hydroxide precipitation. However, with tighter regulations, a two-step precipitation process is now justified but the traditional clarification approach is often infeasible because of the high residence times required which involve substantial cost and space requirements. According to the present invention, the extensive tankage and time requirements of conventional settling techniques are replaced with more sophisticated separation techniques which result in a faster, more space-efficient, and less expensive process.
The present invention describes better ways to do a two-step precipitation that is less costly and requires less equipment than a traditional clarifier yet is able to handle the metal precipitates gently, so as to prevent their breakup.